Among conventional systems having hierarchical structure in which each low-level device is connected with a middle-level device and each middle-level device is connected with a high-level device, a system realizing the collection and management of information on the low-level devices by the high-level device by letting each middle-level device acquire information on the low-level devices and transfer the acquired information to the high-level device is well known, as disclosed in Japanese Patent Provisional Publication No. HEI05-199244 (hereinafter referred to as a “JP 5-199244”), for example.
However, in the system of the JP 5-199244, each middle-level device constantly executes a polling process for checking the presence/absence of the low-level devices, by which the middle-level devices and low-level devices are placed under excessive loads even when collecting the information on the low-level devices only once is enough for the high-level device.
Specifically, each middle-level device, having to periodically execute the polling process even when no low-level device has been added, constantly executes the process wastefully with no low-level device being added, by which the load on each middle-level device becomes heavy. Similarly, each low-level device has to reply to the polling even though the low-level device has already been recognized by the middle-level device, by which the load on each low-level device is also necessitated to become heavy.